The present invention relates to a device transfer method of transferring light emitting devices such as light emitting diodes, which have been formed on a substrate, for example, a sapphire substrate, to a device transfer body such as a display panel, and to a panel on which the transferred devices are arrayed.
A known method of producing an LED (Light Emitting Diode) display using LEDs includes the steps of cutting an LED wafer, which is obtained by stacking semiconductor layers on a device formation substrate, into chips by a diamond blade or the like, and transferring the LED chips to a display panel or the like with a pitch larger than an array pitch of the LED chips on the device formation substrate.
The above-described cutting method, however, has problems. For example, blue light emitting diodes are produced by stacking gallium nitride based semiconductor layers on a sapphire substrate as a device formation substrate. In this case, sapphire used as the material of the substrate is as very hard, about 9 in Mohs' hardness. As a result, if the sapphire substrate is full cut into chips by a dicer such as a diamond blade, problems such as cracking and/or chipping tend to occur in the cut planes of the sapphire substrate which prevent the sapphire substrate from being smoothly cut into chips of desired shapes and sizes, the dicer itself may also be broken, and that since sapphire has no cleavage characteristic, it is difficult to cut the sapphire substrate into chips by forming scribing lines on the sapphire substrate and forcibly cutting the sapphire substrate along the scribing lines by an external force.
To solve the above problems, a method of cutting a sapphire substrate has been disclosed, for example, in Japanese Patent Laid-open No. Hei 5-315646. According to this method, as shown in FIG. 12, a gallium nitride semiconductor layer 61 formed on a sapphire substrate 60 is cut by a dicer to form grooves 62 deeper than a thickness of the gallium nitride semiconductor layer 61. The sapphire substrate 60 is thinned by polishing a back surface of the sapphire substrate 60. Scribing lines 63 are formed on the sapphire substrate 60 via the grooves 62 by a scriber. The sapphire substrate 60 is then forcibly cut into chips by an external force. This document describes how the sapphire substrate can be smoothly cut into chips without occurrence of cracking and/or chipping in the cut planes of the sapphire substrate 50. However, such a cutting method, requires several steps including the labor intensive step of polishing the sapphire substrate 60. In other words, the above method of cutting a sapphire substrate 60 is expensive and time consuming.
If a larger number of LED devices are obtained from one device formation substrate, the cost of one LED device can be reduced and the cost of a display unit using such LED devices can be also reduced. In the method disclosed in the above document, Japanese Patent Laid-open No. Hei 5-315646, LED devices each having a size of 350 μm per side are obtained from the sapphire substrate having a diameter of two inches. If LED devices each having a size of several tens μm per side are obtained from the sapphire substrate having a diameter of two inches and a display unit is produced by transferring the LED devices on a display panel, it is possible to reduce the cost of a display unit.
However, if the size of each LED device becomes as small as several tens of μm per side, it becomes difficult to handle the LED device in the transfer step. Further, since an electrode of each device to be connected to a wiring layer of a base body of a display panel becomes small, the connection work becomes difficult and also a connection failure may often occur.